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Soil Vapor Extraction Test Report for SWMU B-3
Section 5 - Conclusions and Recommendations
There are several conclusions to be drawn from the expanded SVE test data interpretations and observations. Because of the large amount of data collected during this testing event, the conclusions have been separated into two categories. The first category is general conclusions, which discuss items that are related to characterization of the site and other observations that may not be directly related to the effectiveness of the SVE system at SWMU B-3. The second category is conclusions related to the SVE performance.
General conclusions that are associated with characterization of the site are as follows:
Initial (1996) pilot test SVE system operation resulted in significant removal of VOCs in soils located within at least 15 feet from the extraction wells, and perhaps from soils located up to 30 feet from the extraction wells, as evidenced by reductions observed in 1997 confirmation soil samples (Table 4.1). VOC removal was observed primarily in deeper soils in the main trench (Section 4.4.1).
TCE levels were detected above cleanup criteria in two of the 1997 confirmation soil samples (VEW-01 from 9 to 11 feet bgs and MPD from 14 to 15 feet bgs) and in three of the soil samples collected during the 2000 resampling activities (VEW-16 from 18 to 18.5 feet bgs, VEW-17 from 17 to 17.5 feet bgs, and VEW-18 feet bgs from 17 to 17.5 feet bgs). No other organic compounds were detected above the (Section 4.1.2).
The lateral extent of VOC soil contamination in the main SWMU B-3 trench appears to have been defined with the exception of the area south of VEW-18. The area east is defined by VEW-14 and VEW-15, and the area west is presumably limited by the trench boundary. The boundaries of the targeted treatment area were determined by outlining the VEWs that exceeded the RRS 2 cleanup criteria, and including other proximal sample locations with VOC concentrations greater than 10 percent of the cleanup criteria (RRS 2) as a conservative measure. The delineation of the estimated treatment area is shown on Figure 4.1. Cross-sections that depict contaminant concentrations are presented in Figures 2.5, 2.6, and 2.7. Concentration contour maps for TCE, PCE, and cis-1,2-DCE are presented in Figures 4.2, 4.3, and 4.4 (Section 4.1.2).
VOC contamination has migrated into the fractured limestone surrounding the main SWMU B-3 trench, but the areal extent and potential mass of contaminants remaining in limestone fractures is unknown.
The estimated treatment area determined by those portions of the main trench that exceed cleanup criteria is 8,400 square feet, and the total volume of contaminated material in the target cleanup zone is approximately 4,700 cubic yards. This volume estimate assumes that the thickness of contamination is 15 feet bgs and extends from the total depth of the main trench, or 20 feet bgs, to approximately 5 feet bgs. The estimated total mass of contamination in the treatment area is 190 pounds of TCE, and 39 pounds of cis-1,2-DCE. The estimated mass removed during the three MCTs (approximately two months of operation) were 18 pounds of TCE, and 9.7 pounds of cis-1,2-DCE. These removal rates include VOCs located in the target treatment area (main SWMU B-3 trench) and VOCs located in the periphery of this area, such as soils with VOC contamination below RRS 2 and VOCs that have migrated into surrounding bedrock fractures (Section 4.1.2).
Biodegradation of chlorinated volatile organic contaminants has been demonstrated in vadose soils at SWMU B-3. The occurrence of in-situ biodegradation of the VOCs present in the trench will aid in the long-term remediation of VOCs at SWMU B-3 (Section 4.3).
5.1.2 Effectiveness of SVE Operation
The testing activities demonstrated that SVE can be operated effectively at the SWMU B-3 trench and that it can be a valuable mechanism for removal of VOC contamination present in the trench. Several conclusions were drawn from the SVE pilot testing that are specific to the operation of the SVE at the site, and are considered in the development of full-scale design and long-term operation parameters. The conclusions that are specific to the effectiveness of SVE at SWMU B-3 include:
VEW-04 was the extraction well that contributed most to VOC mass removal during the January 1997 system check and during MCT-3. VEW-04 is screened in the limestone located outside the boundaries of the main trench. This conclusion suggests that VEWs installed outside the main treatment area and screened across fractures in the native limestone are capable of exerting the greatest influence on the largest portions of the treatment area (Section 4.2).
The VEWs located in the main trench that showed the highest TVH removal rates during their respective MCTs are VEW-10 and VEW-12 (Section 4.5).
Pulsing the operation of the SVE system would not appreciably improve the efficiency of contaminant mass removal at the site. This conclusion is based on hydrocarbon volatilization rates measured during the hydrocarbon recovery test, and the steady state TVH removal rates observed during the two- to four-week test period for each of the MCTs (Section 4.3 and 4.5).
None of the tested configurations were able to exert any influence on VEW-17, and only the configuration with VEW-03 included was able to influence the soil gas in the vicinity of VEW-18. Since TCE was detected above cleanup criteria in these boreholes, extracting soil gas from the vicinity of VEW-17 and VEW-18 is critical to the long-term success of SVE treatment at this site (Section 4.4).
Extraction efficiencies may be adversely affected by periods of heavy rainfall, as was observed during MCT-2. Results from MCT-2 suggest that flow rates and VOC contamination are located primarily in deeper portions of the main trench, and during heavy rainfall, water levels were found to rise in the VEWs and trench area. Water levels were observed to drop during more arid periods of the performance test. Not only does a rainfall event decrease the available screened area of the formation from which soil gas can be extracted, it also essentially seals the deeper portions of the trench from vapor extraction potential during and immediately after significant rainfall (Sections 4.4 and 4.5).
5.2.1 Establishment of Cleanup Levels and Models
The first recommendation focuses on determining the most appropriate cleanup levels that the SVE remediation system should try to attain. Closure of the site is complicated by the presence of high metal levels in soil samples collected during the pilot test and expanded SVE study activities. The primary objective of SVE remediation is removal of VOC contaminants from a potential source area to protect groundwater from future additional releases. Considering this objective, it may be appropriate to select alternative cleanup criteria based on the VOC levels that can be left in place, such that appreciable leaching of VOCs is greatly minimized or eliminated.
Sufficient data should be available from physical parameter testing to perform modeling to determine a reasonable cleanup level that would minimize any additional releases of VOCs from SWMU B-3. The model should focus on determining the levels in soils that will minimize leaching from the fill material to the fractured limestone. Models which may be appropriate include SESOIL, VLEACH, HELP, or CHEMPLO. Most of the required input data for these models has already been collected.
5.2.2 Full-Scale Design and Operating Parameters
Based on the initial pilot test performed in 1996 and the testing performed on the expanded system in 1997, operation of the SVE system at SWMU B-3 should be continued to remediate VOC contamination present in the SWMU B-3 trench. The following recommendations are for full-scale design and operating parameters regarding the continued operation of SVE at SWMU B-3.
The test system should be expanded to include all 18 VEWs in the extraction configuration. The emission rates observed during the expanded SVE testing indicated that extraction of all 18 VEWs would not cause an exceedance of the standard exemption requirements, so all VEWs could be incorporated into the extraction system without adding expensive off-gas treatment. Vacuum pressures of 25 to 50 inches were applied to each VEW during the three MCTs, and the pressure relief valve, set at 54 inches, allowed atmospheric air volume as make up volume to prevent the blower from overheating. The cumulative flow rates at 50 inches of pressure during each six-VEW MCT averaged approximately 100 cfm during MCT-1, 20 cfm during MCT-2, and 90 cfm during MCT-3. The blower is capable of providing air flow rates of 160 cfm at 65 inches of pressure (maximum vacuum). Whereas the blower may not be sized to optimize removal rates, or to maintain 50 inches of pressure at each VEW, it is possible to decrease the vacuum pressure in less critical VEWs by closing valves, thereby maintaining steady removal from key VEWs.
Once extraction from all 18 VEWs is initiated, flow and TVH levels from each VEW should be monitored and the flow control valves at each VEW should be adjusted to balance the flow. Some VEWs were installed in portions of the trench that appear to contain tighter soils, such as VEW-17 and VEW-18, so it will not be possible to create balanced flow from all 18 VEWs. If maintaining flow from the critical VEWs (locations that are proximal to elevated TCE concentrations) is not possible in an 18-VEW configuration, then the data presented on Tables 4.9, 4.11, and 4.13 should be evaluated to determine which VEWs could be shut off without causing a significant reduction in TVH removal rates.
At a minimum, the critical VEWs that must be included in the full-scale, long-term SVE system include VEW-17, VEW-18, VEW-01, VEW-06, and VEW-13. These VEWs represent portions of the main trench that have TCE contamination data detected above cleanup criteria. Other VEWs that need to be maintained in the full-scale treatment system include VEW-04, VEW-05, VEW-10, and VEW-12. These are the VEWs that exhibited the greatest TVH mass removal rates during the MCTs.
An air emissions standard exemption modification for the expanded system should be prepared assuming use of all 18 VEWs. Any reduction in the number of VEWs actually used in the system would be covered by the 18-VEW system exemption.
Continuous extraction should be used for the operation of the SVE system. TVH removal rates were observed to be fairly constant even after up to 28 days of continuous extraction (MCT-3). The only reason to employ a pulsing scenario would be if insufficient removal rates are observed from any of the critical VEWs when configured with the other critical VEWs. A pulsing scenario may be appropriate to allow separate pull from these areas of the trench. Another alternative would be to add a second blower to the system that would allow simultaneous extraction from two separate VEW configurations.
5.2.3 Improvements for Contaminant Removal
The following recommendations are associated with the potential for improving the efficiency of contaminant removal from the main trench (Figure 5.1).
Installation of additional extraction VEWs in the fractured limestone interval outside, but adjacent to, the main SWMU B-3 trench may be able to improve removal rates from the trench. VEW-04 and VEW-05 demonstrated greater removal capacity than other VEWs installed in the main trench. VEWs located in limestone should also be more effective at removing VOCs that have migrated into the fractured limestone. Soil gas survey data should be used for siting any additional VEWs in the surrounding limestone. VEW-04 and VEW-05 were installed at their existing locations because of high TCE and PCE results. These contaminant levels probably were elevated due to apparent fractures that are interconnected from VEW-04 and VEW-05 to the main trench. New VEW installation in the limestone should focus on other areas outside the trench where high contaminant levels were reported in soil gas.
Additional soil characterization and/or VEW installation may be necessary to define the extent of contamination south of VEW-18 that requires treatment. The spacing and configuration of any additional sample locations should mimic the grid used for the expanded system (Figure 5.1). Additional VEWs should be installed in any additional soil characterization borings drilled at the site and incorporated into the test system for eventual use in the full-scale configuration, if necessary.
A study could be performed to evaluate the influence of rainfall events on water levels in the SWMU B-3 trench, and the rate that water is drained from the trench. This data would assist in understanding the primary extraction zones in the soil and the effects of rainfall on system performance. This data would also be beneficial for refining leachate models for the site. This study should include a review of historic precipitation as well as climatic data during the MCT to determine if any generalization or trends could be identified.
5.2.4 Estimated Costs of SVE Treatment and Recommendations
This section details the estimated costs that have been incurred during the initial 1996 pilot test and the expanded treatability test in 1997. Estimates are also provided for each of the proposed recommendations. The key activities and assumptions used to derive costs for recommended actions are explained in this section.
The estimated costs for the SVE installation and testing activities performed to date are summarized in Table 5.1. The 1996 pilot test cost approximately $80,000, whereas the estimated cost for installation and testing of the expanded SVE system is $152,000. The initial pilot test included installation of six VEWs, six multi-level vapor monitoring points, and procurement of the existing blower and ancillary equipment. CSSA also provided electrical service during the initial system installation, and these costs (about $5,000) are included in this estimate. A two-week treatability test was performed on the initial pilot test, and results of this test were incorporated into a comprehensive groundwater source characterization report (Parsons ES, 1996c).
The expanded SVE system costs included the installation of twelve additional VEWs within the trench limits and the collection of samples for further characterization. The treatability testing activities performed during the 1997 study, which are described in this report, included three multiple configuration tests and a hydrocarbon recovery test that spanned four months. Soil gas/emissions samples were collected for laboratory testing during each of the testing events. The reporting costs are based on anticipated level of spending at final submittal of this report.
The recommendations from Sections 5.2.2 and 5.2.3 are summarized in Table 5.2, with rough estimations of costs to implement each recommendation. Plan preparation and reporting costs are not included in the estimate for any of the listed recommendations.
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